Pump Affinity Law — Power vs Speed
Worked example: 15 kW at 1450 rpm slowed to 1160 rpm → 7680 W — press Try an example to run it live, then adjust anything.
Enter your known values, leave one input blank, and solves for the missing one. Tap a variable’s symbol to see what it means, with a typical value. Try different units for next level excitement!
Affinity with speed →
UniversityFluid Mechanics, HVAC & Refrigeration
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Pump Affinity Law — Power vs Speed explained
Power is head times flow, and head already carries a square, so power rides the cube of speed. Slow a 15 kW pump from 1450 to 1160 rpm — a mere 20% cut — and it draws 15 × 0.8³ = 7.68 kW, a 49% saving. That cube is why throttling a valve to reduce flow is such a waste: the pump still spins at full speed burning full power while you dissipate the surplus as heat and noise across the valve seat.
Two cautions temper the arithmetic. Real pump and motor efficiency sags at part speed, so the field saving is nearer 45% than 49%, and any static lift in the system means the pump cannot follow the cubic curve all the way down. Chilled-water plants still routinely bank 40–60% on pumping energy, which is why ASHRAE 90.1 has effectively mandated variable-speed pumping on larger variable-flow systems.
Pump Affinity Law — Power vs Speed formula
- = Power at speed 1 (W)
- = Speed 1 (rpm)
- = Power at speed 2 (W)
- = Speed 2 (rpm)
Missing one of these? Work it out first, then come back
- Power at speed 1 — Fan Affinity Law — Power vs Speed, Pump Efficiency from Hydraulic and Shaft Power
- Speed 1 — Pump Affinity Law — Flow vs Speed, Pump Affinity Law — Head vs Speed
- Power at speed 2 — Fan Affinity Law — Power vs Speed, Pump Efficiency from Hydraulic and Shaft Power
- Speed 2 — Pump Affinity Law — Flow vs Speed, Pump Affinity Law — Head vs Speed